you should be able to find it and change it by looking at "my tax data"

Windows

From the TurboTax Tools menu, select My Tax Data.In the My Tax Data window, the Source column will indicate whether the data on the corresponding Form and Line was transferred, imported, or entered by you.(Optional) Click the Source column heading to group and alphabetize the list.Select the line item you want to view or change, and then click the Jump To Interview or Go To Form to view, change, or remove the line item in Interview or Forms Mode, respectively.Mac

At the top of the program window, press the Tax Data button.In the My Tax Data window, the Type column will indicate whether the data on the corresponding Formset/Form/Field Name was transferred, imported, or entered by you.(Optional) Click the Type column heading to group and alphabetize the list.Select the line item you want to view or change, and then click the Jump To Interview or Go To Form to view, change, or remove the line item in Interview or Forms Mode, respectively.

sorry. to me it seems like you are just not hitting the correct section to put the dividend in the same way. But I don't think intuit has as many features in the online version as the desktop and I suspect they don't have quite the bug checking for the Canadian version as they do for the American, so it may be you just hit a bug they didn't notice and there is no other way to do it.

Done that. No dice. The problem is that I had entered it as dividends not covered by a T5 slip. Once it's entered, it seems to "disappear", except for showing up in the Income Total and in the tax calculation.

sorry, In the American version all of that data is available under my tax data but I am not that familiar with the Canadian version and if the people at intuit could not figure it out my guess is that it is just a bug with the program and nobody remembered to put in a way to change that particular field

In that case you would have to re do the entire form. I would suggest printing out a copy before you do that and that would make it easier to re input.